It is generally well recognised that it is useful to be able to disconnect a rotating machine, such as a generator, from its drive source or prime mover in the event of a fault developing within the machine. Where the fault is electrical, sensing equipment may cause electrical isolation of the generator. However where the fault is mechanical, for example the loss of lubricating or cooling oil or failure of an associated lubricating or cooling oil pump causing the generator to fill with oil, then the continued rotation of the machine may cause overheating resulting in catastrophic failure or a fire. If the generator is an aeronautical or aerospace generator, then this situation could result in damage to or impairment of the functionality of the engine or gearbox driving the generator. Therefore mechanical protection systems are required.
U.S. Pat. No. 5,103,949 discloses a protection system in which a generator receives its drive by a drive arrangement comprising two drive shafts which are coupled together via inter-engaging dogs. One of the shafts carries a screw threaded region 20 and is biased into engagement with the other shaft by a compression spring. In the event that it is desired to disconnect the drive, a spring loaded plunger is driven into a position where it can engage the screw threads and therefore give rise to a unscrewing action which separates the shafts from mechanical inter-engagement. This mechanism is relatively complex and increases the axial length of the generator and drive arrangement. Furthermore the disconnect mechanism is quite complex and hence will be relatively heavy.
U.S. Pat. No. 4,392,835 discloses an alternative system in which a tungsten carbide cutting blade is spring loaded such that it can be moved into engagement with the drive shaft when it is required to disconnect the generator. The shaft in the region of the cutting tool is formed as a relatively thin walled tube. Friction causes local heating and the relatively thin wall of the shaft becomes plastic, allowing the shaft to separate into two portions thereby disconnecting the drive to the generator. A concern with this arrangement is that it relies on the spring pressure on the cutting tool being great enough to cause the required shaft heating and subsequent failure. Therefore this disconnect mechanism may not operate efficiently in all circumstances. For example, the shaft may be rotating at a high speed and may be in an oil filled and/or cooled environment. The required frictional heating may take some time to develop. Furthermore, if the actuating spring is large enough to ensure that there will always be sufficient pressure to break the shaft then it will be large and relatively heavy and will require a large force to be generated by a trigger mechanism in order to release it.
Similar “shaft cutting arrangements” are described in U.S. Pat. No. 2,862,375 and U.S. Pat. No. 3,427,826.
It is common practice to provide a shear neck, that is a region of reduced thickness where structural failure will occur in the event of excessive loading, to protect the prime mover and/or gearbox from damage. The shear neck is provided irrespective of whether or not some other disconnect mechanism is also included. U.S. Pat. No. 5,418,412 shows an arrangement in which the provision of the shear neck is utilised. U.S. Pat. No. 5,418,412 is primarily concerned with a generator in which there is a possibility that an oil scavenge system may become incapacitated, thereby resulting in flooding of the generator. This could cause severe damage to the generator. In order to overcome this problem, a fluid brake in the form of an impeller is attached to one end of the generator shaft. If the generator starts to flood the impeller interacts with the oil to generate a large drag torque. This drag torque is greater than the shear torque engineered into the shear region and therefore causes the shear neck to fail thereby disconnecting the generator from the prime mover.
U.S. Pat. No. 3,620,046 discloses a shaft having one or more shear necks and also carrying an integrally formed part conical disc region of enlarged radius. A coaxially mounted brake component is mounted such that upon release of a latch mechanism (latch pin 25) the brake component moves under the urging of compression springs so as to engage with the disc region. This gives rise to a braking force acting on the disc leading to failure of the shear neck so as to disconnect the drive. The moving brake component is mounted to a housing of the device by inclined keys which serve to increase the application force between the brake component and the disc in response to torque caused by the frictional inter-engagement of these components. A significant disadvantage of this design is the large contact area between the moving and stationary parts. Unless the springs are very strong there may not be sufficient “bite” to enable the non-rotating component to “latch” onto the rotating part. The presence of lubricant only makes this problem worse. The mechanism is also relatively bulky and heavy.
GB 1044094 discloses a disconnect device in which loading of a shaft causes failure of a shear neck. The loading mechanism comprises a flange having axially extending projections (14). A spring loaded pin is held within a housing and can be moved axially to engage with the projections. A concern over this design is that high speed rotation of the shaft could result in the pin either bouncing off the projections or being worn away by successive contact.
U.S. Pat. No. 3,220,218 discloses a disconnect mechanism in which a cam is helically threaded on a drive shaft. A disconnect element is moved to a position so as to engage one of the radially extending surfaces of the cam thereby stopping the cam from rotating. The cam then moves along the thread until it engages an end stop where the shaft is loaded to cause a shear region to fail. Again there is a risk that high speed rotation of the shaft could cause the disconnect element to bounce off the cam rather than operating the disconnect mechanism as intended.